The present invention stems from the observed fact that once the allotropic transformation of austenite in medium to high carbon content steel has started to take place in a given portion of an elongated steel member which is being cooled non-uniformly, transformation in the adjacent warmer portions of the steel is "sympathetically" triggered and transforms sooner, all other things being equal. This is particularly noticeable in steel immediately after hot rolling and cooling when the cooling is done sufficiently soon after rolling to retain relatively small austenite grains (i.e., in the range of ASTM 6-9). Thus, in the well-known process described in U.S. Pat. No. 3,231,432), when rolling medium to high carbon steel, if one stations himself alongside the conveyor at the appropriate place, one can "see" the transformation start, usually at the center of one ring and proceed rapidly along the rod toward the hotter portions of the rod. What one sees is actually a change in color of the rod from nearly black to red, due to the recalescence of transformation. Thus, in the first parts of the rod to reach transformation, the temperature has descended to a nearly black condition (about 600.degree. C. to 650.degree. C.) and immediately as transformation progresses they turn red again (about 750.degree. C. or possibly higher). Thus, it appears that during cooling, the steel reaches a super-cooled state and when transformation is finally triggered a more-or-less violent release of heat takes place. Thereafter it appears that the triggering proceeds rapidly along the rod and transformation starts elsewhere without the same degree of super-cooling or the same violence of recalescence. This is particularly true when relatively small austenite grains in a highly uniform state are involved. Thus, with such a structure, the transformation conditions for each successive grain are virtually the same, and the triggering chain-reaction is not blocked by the presence of non-conforming grains as occur for example in the mixed grain size structures obtained in typical steel products processed by reheating about A.sub.3 and cooling alone.
The foregoing observations actually serve as a basis for understanding why, in the process of U.S. Pat. No. 3,231,432, a relatively uniform product can be obtained even though various parts of the rod are clearly being cooled at very non-uniform rates. Transformation starts at the coolest portions first and proceeds along the rod toward the hotter portions where it triggers the transformation, before those portions reach a super-cooled condition. Transformation proceeds relatively rapidly throughout the rod due both to the triggering chain-reacting and to the smallness of the austenite grains. Thus, the formation of excessive free ferrite is avoided throughout the rod even in the places where the rings overlap and appear to be transforming at a much slower rate. In fact, in the edge areas where the rings overlap and form massed groups, the rod remains red hot continuously, and substantially less recalescence is observed. It is believed, however, that even though the rod is still red hot, the structure has already been effectively transformed at this stage, at least in the sense of inhibiting the further formation of free ferrite, and that this is due to the sympathetic triggering reaction of transformation in adjacent parts of the rod. The result is, therefore, a relatively uniform product despite the obvious non-uniformity of the cooling rate in various parts of the rod.